Commercial and domestic fire detectors are usually one or more of three types. This assumes your project is some sort of warning device?
Optical for smoke particles.
Thermal for heat.
Ionisation for the products of combustion.
I guess you are looking at the flame which is probably not the best approach if you want a warning when the flame is somewhere else.
I remember some instrumentation, flame photometric detectors, that looked at changes in the colour of the flame, where either sulphur or phosphorous was present. Other devices are mainly for flame failure.
Why not try several sensors? You can get visible and IR photodiodes with built in IR filtering and an op amp as well. I think Sharp used to do them as well as Texas.
The op amp makes them very easy to use with a voltage analog output.